Television camera tube



l May A149, 1959 P. SCHAGEN 2,887,611

TELEVISION CAMERA TUBE Filed July 21. 1954 CcLLEcj-oe.

GLAss -r'AzGz-r gain): A mmmmmmmm INVENTOR PIETER SCHAGEN AGENT UnitedStates Patent O TELEVISION CAMERA TUBE Pieter Schagen, Eindhoven,Netherlands, assigner, by

mesne assignments, to North American Philips Company, Inc., New York,N.Y., a corporation of Dela- Ware Application July 21, 1954, Serial No.444,898

Claims priority, application Netherlands August 4, 1953 3 Claims. (Cl.315-11) This invention relates to television camera tubes.

Some television camera tubes include an image electrode made frommaterials having a relatively high specic resistance, These materialsare termed semi-conductors and have the property that, under the inuenceof a potential diiierence, charge carriers are transferred from aconducting coating provided on onesurface of the electrode to itsopposite side on the surface of which the luminous image is projected,or where, with a photoelectric cathode converting the luminous imageinto an electron image, electrons are focussed.` Furthermore, saidsurface is scanned by an electron beam of small cross-sectional area atits point of impact. resulting photoor secondary emission, electronsfaredislodged from the image electrode and carried oit to an adjacentcollector electrode.

The present invention relates to a camera tube of the type wherein thecollector is positiveA with respect to the cathode of the electrodesystem producing the scanning beam, and the electrons of this beamreaching the image electrode at a high velocity dislodge va number ofsecondary electrons larger in number than that of theelectrons of thescanning beam. The operation of such a tube is based on the potentialshift, due to electron emission, ofthe electrically-heating surface ofthe image electrode.

The value of the electric charge perv image element of the electrodesurface and the strength of thesignal obtained by means of the scanningray, `depend .upon the quantity of electrons carried olf at the impactside of the image electrode during the time between two scans. Byapplying a negative voltage to the conducting coating on the rear sideof the image electrode relativeto the collector, the strength of theelectric field required for carrying oi said electrons to the collectorcan beset to a value at which all the electrons are collected. However,raising the eld strength in order to remove thev secondary electrons atthe impact side of the image electrode isopposed or ofset by the factthat` under the influence of the potential ditference between theconducting rcoating and the collector electrode, image charge is lostdue to the conductivity of the image electrode. Further, an increase inthe collecting eld strength for the electrons means a larger shift ofthe surface potential at the impact side of the image electrode underthe influence of the scanning beam, and, moreover, more charge losses inbuilding up the potential pattern, which is an electric translation ofthe image to be transmitted.

It is known that the application of a negative voltage to the surface ofthe image electrode provided with a conducting coating with respect tothecollector is advantageous for avoiding disturbing signals. vThelatter are produced by secondary electrons returning to the-surface ofthe image electrode due to space charge phenomena and beingredistributed thereover. In the known arrangement, the eiect is suchthat the potential uctuations produced at the image electrode surfacebythe scanning beam do not cause the surface 'potentialat the impact sideof theimageelectrode to be larger-than is`'permissible''forv Due to themaintaining the field strength required for carrying off substantiallyall dislodged electrons to the collector. This condition limits thechoice of the intensity of the scanning beam to that required to avoidthe disturbing signals. Further, it has been found that there is adependency existing between the potential setting, the exposure strengthand the inertia with Which the potential setting passes over to adifferent value, thereby causing disturbances in reproducing rapid imagevariations. A rapidly moving object is not clearly reproduced in theimage, and the image of a subject swiftly removed from the image eld issuppressed too slowly.

In accordance with the invention, this disadvantage is mitigated byequipping a television camera tube with an image electrode made from amaterial having a high specic resistance and whose thickness is at leastone third of the size of an image element and is not more than threetimes its size, the specific resistance of the material being at leastl011 ohm-cm. with the lower value of said lthickness and having acorrespondingly higher value with a greater thickness.

When using the invention, the limitation with respect to the intensityof the scanning beam is dispensed with. In lthis case, the advantagethat electrons from the image electrode surface struck by the scanningray do not return to this surface is lost; however this is not a seriousobjection because the infiuence of said stray electrons is greatlyreduced, since the disturbance which they may bring about depends uponthe charge-absorbing capacity of the image elements. This is due to thefact that as the capacity of the image elements with respect to theconducting coating at the back of the image electrode is reduced, or,alternatively, as the thickness of the semiconductor increases, lesscharge is required to produce a given potential variation at the surfaceof the image electrode. Since a necessary potential Variation may beassumed as that at which no further stray-electrons are absorbed by animage element, a prerequisite is that the negative voltage set up at theconducting coating of the image electrode shall be much higher so thatin producing the image charges for obtaining the potential pattern,despite returning secondary electrons to the image elements, thepotential-reducing iniluence of the negative voltage set up at theconducting coating is sufcient yto bring the image elements into saidstate Without any appreciable delay. Due to the low capacity per imageelement, the charge required therefor is so 10W as to prevent anyappreciable disturbing signals. The potential stabilization of thesurface of the image electrode by the scanning beam has a constant Valuesubstantially corresponding to the collector Voltage, which value isindependent of the degree to which the surface is struck byimage-producing rays. After each scan, during which an image elementattains a xed potential, the potential is varied under the' influence ofthe image-producing rays, and the electron.

supply from the conducting coating and the variation during the timeelapsing between two scans is a measure of the image signal strength.

The choice of the specific resistance of the material from which theimage electrode is made is related with the loss of image charge due todiverting electrons through the material, the displacement of chargecarriers between two image elements at different potentials also havingto be considered. The last-mentioned charge losses increase with thethickness of the electrode, while the image signals should not beweakened to such a degreeas to offset the advantage realized from thepotential shift.

The invention will nowbe described, by way of example, With reference tothel accompanying drawing',

which represents 'a t camera tube V.of the image-iconosccpe;

3 type according to the invention with means for supplying the requiredvoltages.

Referring to the drawing, there is shown a substantially cylindrical,glass wall 1, of a camera tube. The larger` diameter part of the wall isclosed by means of a glass` bottom 2 including a supporting insulator 3through which a metal support 4 for the image electrode or electronimage receiving electrode 5 is led.

A part of smaller diameter of the Wall 1 is closed by means of a atbottom 6 carrying a photo-electric cathode 7. The latter is made fromlight-sensitive material emitting photoelectrons under the influence oflight rays. This photo-electric cathode may be of the usual kcom--position so that it will not be further described.

A laterally-projecting tube S contains an electrode system 9 producingand controlling the strength of a scanning beam whose axis extends inthe direction of the center of the image electrode 5. Detiection coils10` and 11 supply the elds required for varying the path of. thescanning beam so that it may be directed to any point of `the imageelectrode.

The Wall of the tube is provided with a conductive coating 12 which iselectrically connected to a collector- 13 arranged adjacent the imageelectrode S.

The image electrode 5 is made from semi-conducting material and, at theside remote from the photo-electric cathode 7 and the electrode system9, is provided with a conducting layer 14. The specific resistance ofthe semi-conducting material from which the electrode is made is atleast 1011 ohm-cm. Use may be made of vitreous enamel provided on ametal plate. Glass species are known which approximately have saidspecific resistance and can be worked to form a thin layer of uniformthickness and homogeneous composition. A glass plate having the requiredthickness can be made suiciently smooth by grinding and polishing. Thecomposition of the enamel corresponds to that of a suitable glass andmay be made from a glass mix or melt comprising mainly silica and sodiumoxide together with the oxides of potassium and aluminum.

Glass having the desired characteristics has been made from a mixcomprising the following specilic component quantities in percent byweight Silica 69.80 Sodium oxide 18.00 Aluminum oxide 2.30

Potassium oxide 8.80 Zinc oxide 0.50 Manganese oxide 0.60

The conducting 'layer 14 is connected through a resistor 15 to thenegative terminal of a source 16 whereof the positive terminal isconnected to the wall coating 12. The collector 13 has the samepotential as the coating 12 and is thus biased positively relative tothe conducting Llayer 14. A second source 17 delivers suitable voltagesfor producing the scanning beam and is connected to the photo-electriccathode 7. An accelerating voltage is applied to the wall coating 12 andthe collector 13 so that the electrons of the scanning beam impinge at ahigh velocity on the surface of the image electrode S, in-as-much as thevoltage supplied by the source 16 is small compared with theaccelerating voltage from the source 17.

After the scanning beam has several times swept the impact surface ofthe image electrode, the whole surface assumes an average potentialwhich depends upon the voltage between the conductive coating and thecollector, the beam current, the resistance of the semi-conductor andthe secondary-emission coetiicient of the surface struck. It isadvisable to improve the electron-emission properties of the surface ofthe semi-conductor material and to this end an oxide of an alkaliorearth-alkaline-metal, which possesses a high secondary-emissioncoefficient,

may be spread in the form of a thin layer overthe sur-` face. The nextscans raise the potential of an image element approximately to thecollector voltage, the potential reassuming its average value throughelectron-conduction through the image electrode 5 between two scans.

During exposure between two scans, in general photoelectric emission,and in particular for the image iconoscope shown, furthersecondary-emission will occur due to the image-producing rays impactingthe photo-cathode 7 and releasing photo-electrons that bombard the imageelectrode 5, and a part of the charge carriers, supplied throughconduction from the conductive coating to the image elements, will beremoved. In this case, the potential shift occurs less rapidly than inthe absence of light incident on the photo-cathode. During the availabletime period, the potential of the elements drops to a lesser degree, andthis drop of potential to a greater or lesser degree is a measure of thedifferences in signal strength of the individual image elements. A videosignal may be derived across the resistor 15 when the highvelocityscanning beam restabilizes the potential of each image elementapproximately at the collector potential.

As pointed out hereinbefore, the thickness of the semiconductor materialis between one-third and three times the diameter of an image element,the latter of which is dependent on the scanning rates and the overallsize of the image electrode. Generally, the thickness of thesemi-conductor will range between 0.034 and 0.3 mm.

While I have described my invention in connection with specificembodiments and applications, other modications thereof Will be readilyapparent to those skilled in this art without departing from the spiritand scope of the invention as defined in the appended claims.

What is claimed is:

l. A television camera device of the image-iconoscope type comprising aphoto-electric cathode at one end, an electron-image-receiving electrodeat the other end, said image electrode comprising a conductive memberand a semi-conductive glass material facing said cathode and having aspecific resistance of at least 1011 ohm-cm., a collector disposedadjacent said image electrode and between the latter and said cathode,means for biasing said `conductive member at a relatively low negativepotential with respect to said collector, means producing ahigh-velocity electron beam for scanning said image electrode as aplurality of discrete image elements of given size and generating more`secondary electrons than primary electrons for stabilizing thepotential of said image elements at a positive value in the vicinity ofthe collector potential, said semi-conductive material having athickness between 1/3 and 3 times the size of an image element on saidimage electrode scanned by the high-velocity beam and ysaid thicknesslying in the range of 0.034 to 0.3 millimeter whereby the positivestabilized potential of each image element falls ot and approaches thenegative potential of the conductive member as a consequence ofcontinuous current flow through the semi-conductive image electrodethroughout the entire time interval between the high-velocity beamscansions, the number' of photo-electrons from the cathode impactingeach image element and producing further secondary electrons limitingthe image elements potential drop and determining its iinal value, andmeans coupled to said conductive member for deriving an electricalsignal therefrom depending on the potential increase acquired by eachimage element when impacted by the high-velocity scanning beam.

2. A television camera device of the imageconoscope type comprising aphoto-electric cathode at one end, an electron-image-receiving electrodeat the other end, said image electrode comprising a conductive membercontacting a semi-conductive glass member facing said cathode and havinga specific resistance of at least 1011 ohmcm., a layer of materialpossessing a high secondaryelectron-emission coefficient on said glassmember, a collector disposedadjacent said image electrode and sur- 5rounding a portion of the space between the [latter and said cathode,means for biasing said conductive member at a relatively low negativepotential with respect to said collector, means producing ahigh-velocity electron beam for scanning said image electrode as aplurality of discrete image elements of given size and generating moresecondary electrons than primary electrons for stabilizing the potentialof said image elements at a positive value in the vicinity of thecollector potential, said semi-conductive glass member having athickness between 1/3 and 3 times the size of an image element on saidimage electrode scanned by lthe high-velocity beam and said thicknesslying inthe range of 0.034 to 0.3 millimeter whereby the positivestabilized potential of each irnage element falls on. and approaches thenegative potential of the conductive member as a consequence ofcontinuous current ow through the semi-conductive image electrode in thetime interval between the high-'velocity beam scansions, the number ofphoto-electrons from the References Cited in the tile of this patentUNITED STATES PATENTS 2,150,160 Gray Mar. 14, 1939 2,177,736 Miller Oct.31, 1939 2,518,434 Lubszynski Aug. 8, 1950 2,747,131 Sheldon May 22,1956 2,747,133 Weimer May 22, 1956 2,776,387 Pensak Ian. 1, 1957

